The present invention relates to methods of colour balance and means for use in colour balance in colour video cameras and display systems.
In colour video systems, colour balance is a critical factor in perceived image quality. Colour balance is the process of weighting the individual colour channels of a composite colour system, e.g. red, green, blue (R, G, B) to achieve the greatest fidelity of the image compared to the original scene. Normally the key objective of colour balance is for a white object to be imaged with equal energies in R, G, B. This is conventionally achieved by methods and apparatus based on white colour luminance which is widely and successfully used in modern cameras.
In general this conventional approach uses a separate white balance sensor containing relatively few pixels which receives diffused light through a white diffuser direct from the field of view so as to provide what would be expected to be a white image. Any deviation from a white output, i.e. with substantially equal red, green, and blue signal levels, from this white balance sensor is then corrected by adjusting the individual channel gains. The corresponding channel gain adjustments are then applied to the main image sensor.
This system does however have the disadvantages of requiring a separate additional sensor, associated processing means, and means for transferring colour balance information to the main image sensor output. These in turn result in additional manufacturing complexity and cost.
It is an object of the present invention to avoid or minimize one or more of the above disadvantages.
We have found that colour balance can be successfully achieved by a new approach using highlight areas in the green signal. This is particularly effective as maximum green signal levels occur in a number of different principal highlight colours viz white, yellow, cyan and green. Red and blue peak signal levels are also monitored at areas of the image where green is at its peak level. The monitored red and blue peak levels are compared with the green peak level. If any red or blue peak levels are greater than the corresponding green peak levels in an image, then the image is judged to be imbalanced and the gain of the imbalanced red or blue channel is decreased until the difference is substantially eliminated. Because the technique has been extended to be able to judge colour balance on scenes from white to yellow, as well as cyan and green, the monitored lower peak level of the red or blue channels may not represent their real peak level in the image: for example, the blue peak in a yellow image highlight and the red in a cyan highlight. The lower peak level could be caused either by colour imbalance or by a shortage of that colour in the scene.
To solve this further problem the monitoring area for red and blue peak values may be extended over the whole image rather than just at the green highlights. This measurement will then have much higher probability of representing correct peak colour levels in the image so that the gain can be increased until the monitored peak level for each colour becomes substantially equal to the green peak level.
These techniques can be used to realise a real time digital auto-colour balance controller by monitoring the three colour channels without the need for the secondary apparatus or data handling means as required by conventional white luminance colour balance systems thereby facilitating a simple and/or more economic form of manufacture.
It will also be understood that the present invention is not restricted to the RGB system but may also be used in other composite colour systems having a first colour predominantly associated with or characteristic of highlight areas and at least two further colours, e.g. the Cyan, Magenta, Yellow colour system in which the yellow colour is considered to be characteristic of highlight areas.